Tieing into central vac exhaust for fan exhaust?

504 Views 12 Replies 5 Participants Last post by user_12345a, May 11, 2020

turbine

Discussion Starter · May 11, 2020

Hi all. I'm in the process of renovating my garage, and one item on the wish list is an extraction fan. Nothing special, just a standard bathroom fan. I'd like to place it in the ceiling, in the center of the garage. My concern is running the vent pipe. It would require cutting holes in the joists. So I had a crazy idea.

I already have a central van installed in the garage that vents to the outside via a 2" OD PVC pipe. So I thought, why not tie into this for the fan's exhaust? To prevent back-feed I could install a check valve upstream in each pipe.

Does this idea pass the sniff test? I would of course need to reduce the 3" fan exhaust to 2". Also not sure if I'll have a problem if both the fan and vacuum are active.

Thoughts?

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Nealtw

· #2 · May 11, 2020

:biggrin2

ut your fan on an outside wall and put that ingenuity to better use.:biggrin2:

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turbine

Discussion Starter · #3 · May 11, 2020

Nealtw said:
:biggrin2ut your fan on an outside wall and put that ingenuity to better use.:biggrin2:

Haha, yeah I suppose it would

But then again, building inspectors have a real talent for finding new wall penetrations. I'm scared of those guys :biggrin2:

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Nealtw

· #4 · May 11, 2020

turbine said:
Haha, yeah I suppose it would But then again, building inspectors have a real talent for finding new wall penetrations. I'm scared of those guys :biggrin2:

If you down size the duct the fan would as load as an engine.

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bfrabel

· #5 · May 11, 2020 (Edited)

turbine said:
Haha, yeah I suppose it would But then again, building inspectors have a real talent for finding new wall penetrations. I'm scared of those guys :biggrin2:

You should be scared. They are out prowling the world for stuff that's hacked in and crazy. By the sounds of it, I'd keep them far, far away from your place.:vs_cool:

As far as what Nealtw said about how pinching the exhaust fan down would make it more noisy, I don't think it would. In fact, since it would be moving so little air it would probably barely make any noise at all. It would also use less electricity (due to it having to do less work), so I guess that's a plus too!

I would suspect that the bathroom exhaust fan would not have enough power to open up a PVC check valve, since most of them are made for water, not a farts worth of air. So it would just be sitting there spinning and not doing much.

The type of check valves that these fart fans are made to operate, are the kind that are built into the caps that go on the roof. Have you considered something like that?

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Nealtw

· #6 · May 11, 2020

bfrabel said:
You should be scared. They are out prowling the world for stuff that's hacked in and crazy. By the sounds of it, I'd keep them far, far away from your place.:vs_cool:

As far as what Nealtw said about how pinching the exhaust fan down would make it more noisy, I don't think it would. In fact, since it would be moving so little air it would probably barely make any noise at all. It would also use less electricity (due to it having to do less work), so I guess that's a plus too!

I would suspect that the bathroom exhaust fan would not have enough power to open up a PVC check valve, since most of them are made for water, not a farts worth of air. So it would just be sitting there spinning and not doing much.

The type of check valves that these fart fans are made to operate, are the kind that are built into the caps that go on the roof. Have you considered something like that?

Motors are stupid, they don't know that you put a restriction in front of it. It will work hard to get up to speed and over heat. The fan will chop the air instead of moving it. If you have ever had a noisy bathroom fan the first suggestion is bigger ducts and less elbows (less restriction)

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lenaitch

· #7 · May 11, 2020

I'm not sure a typical bathroom-sized fan will do much to effectively ventilate a garage. Also, I think you will get a venturi effect. As air speed increases, its pressure drops, so when the the central vac is running it will want to suck air out of the garage. How big a problem that is, I don't know. I vote for separate systems.

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bfrabel

· #8 · May 11, 2020 (Edited)

Nealtw said:
Motors are stupid, they don't know that you put a restriction in front of it. It will work hard to get up to speed and over heat. The fan will chop the air instead of moving it. If you have ever had a noisy bathroom fan the first suggestion is bigger ducts and less elbows (less restriction)

I was waiting for someone to question what I said about the fan moving less air. Here's the deal... With a squirrel cage type fan (which I believe most bathroom exhausts are), if you restrict the airflow the fan will spin faster with less restriction. As you said, the blower is "chopping the air", or cavitating, which means it is basically free spinning without doing much work. Therefore, it will work less hard to get up to speed. You were right about the motor possibly overheating though, but it would be because of less air flowing across it, not because it's working too hard.

If you were to take this same fan with the restricted ductwork, and then disconnected the ductwork and compared the amp draw, it would draw much more amperage with no ductwork. This is because the air has weight to it and the more air it has to scoop up and move, the more work it is for the motor.

With this being said, propeller type fans don't follow this logic. If you took a propeller fan and blocked the airflow, the motor would strain and amp draw would go up.

If you had an amp meter and a furnace you could check this out. Check the amp draw with a dirty air filter, and then remove the air filter and check it again. Amp draw will be less with the dirty filter and more with no filter.

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Nealtw

· #9 · May 11, 2020

bfrabel said:
I was waiting for someone to question what I said about the fan moving less air. Here's the deal... With a squirrel cage type fan (which I believe most bathroom exhausts are), if you restrict the airflow the fan will spin faster with less restriction. As you said, the blower is "chopping the air", or cavitating, which means it is basically free spinning without doing much work. Therefore, it will work less hard to get up to speed. You were right about the motor possibly overheating though, but it would be because of less air flowing across it, not because it's working too hard.

If you were to take this same fan with the restricted ductwork, and then disconnected the ductwork and compared the amp draw, it would draw much more amperage with no ductwork. This is because the air has weight to it and the more air it has to scoop up and move, the more work it is for the motor.

With this being said, propeller type fans don't follow this logic. If you took a propeller fan and blocked the airflow, the motor would strain and amp draw would go up.

If you had an amp meter and a furnace you could check this out. Check the amp draw with a dirty air filter, and then remove the air filter and check it again. Amp draw will be less with the dirty filter and more with no filter.

I was thinking propeller type and I have not heard that about squire cages before. interesting. :wink2:

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user_12345a

· #10 · May 11, 2020

2" pipe is far too small for exhaust fan and doing what you want could wreck havoc with the vacuum.

do a separate exhaust pipe.

the typical 50 to 70 cfm bathroom exhaust fan won't cut it.

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turbine

Discussion Starter · #11 · May 11, 2020 (Edited)

I agree with everyone's comments. The question boils down to whether a basic bathroom fan can generate enough pressure to open the check valve. According to the spec on this valve the minimum opening pressure is "less than .5 PSI". That really doesn't seem like much, no? Obviously bathroom fans aren't meant to generate much pressure, but maybe after I throttle the 3" output to 2", it'll be enough.

Edit: Although not explicitly mentioned, that valve I believe is intended for liquids. It's max flow rate is rated at 5 ft/s. If I assume 150 CFM output for the vacuum, this translates upwards of 30 ft/s of air. Not sure if the relationship liquid to air is 1:1.

I had not thought of the venturi effect. I don't think that would cause too much of an issue though. Well, unless the fan is running when the vac is active, in which case it may end up spinning the motor and creating a generator effect? Hmmm. I wonder if that could damage the fan motor.

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turbine

Discussion Starter · #12 · May 11, 2020

user_12345a said:
2" pipe is far too small for exhaust fan and doing what you want could wreck havoc with the vacuum.

do a separate exhaust pipe.

the typical 50 to 70 cfm bathroom exhaust fan won't cut it.

Would this really affect the vacuum though? It pumps out a lot air to create suction. Or do you mean if I'm running both at the same time? If the latter, yeah I can see that being an issue.

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user_12345a

· #13 · May 11, 2020

running both at the same time.

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